1. Field of the Invention
The present invention relates to a structure of a display device for increasing a contrast ratio.
2. Description of the Related Art
A display device which is very thin and lightweight as compared to conventional cathode-ray tube display devices, a so-called flat panel display, has been developed. A liquid crystal display device including a liquid crystal element as a display element, a display device including a self-light emitting element, an FED (Field Emission Display) using an electron source, and the like compete in the market of flat panel displays. Therefore, lower power consumption and a higher contrast ratio are required in order to increase the added value and differentiate from other products.
A general liquid crystal display device is provided with one polarizing plate on each of substrates to keep a contrast ratio. Contrast ratio of the liquid crystal display device can be increased by decreasing black luminance thereof. Therefore, higher display quality can be provided when images are seen in a dark room such as a home theater room.
For example, they have proposed the structure in which a first polarizing plate is provided on the outer side of a substrate on the viewing side of a liquid crystal cell, a second polarizing plate is provided on the outer side of the substrate opposite to the viewing side, and the third polarizing plate is provided for heightening the polarization degree when light from an auxiliary light source provided at the end of the substrate opposite to the viewing side is polarized through the second polarizing plate and passes the liquid crystal cell, in order to increase a contrast ratio (Reference 1: PCT International Publication No. 00/34821). As a result, it is possible to suppress unevenness of display and improve a contrast ratio, which are caused due to shortage of polarization degree and polarization distribution of polarizing plates.
The viewing angle dependence of contrast ratio has been regarded as a problem. A primary factor of viewing angle dependence is optical anisotropy in accordance with the long axis direction and the short axis direction of liquid crystal molecules. Due to optical anisotropy, the visibility of an image which is looked at is different to their visibility when looking at the device from an oblique direction. Consequently, the luminance of white display and the luminance of black display change depending on the viewing angle, and the contrast ratio also has viewing angle dependence.
In order to solve the problem of the viewing angle dependence of the contrast ratio, a structure in which a retardation film is inserted has been proposed. For example, in vertical alignment mode (VA mode), by setting up biaxial retardation films having three directions having different refractive indexes so as to interpose a liquid crystal layer, the viewing angle dependence is suppressed (Reference 2: ‘Optimum Film Compensation Modes for TN and VA LCDs’, SID98 DIGEST, p. 315-318).
Further, a structure employing stacked wide view (WV) films in which a discotic liquid crystal compound is hybrid-aligned has been proposed for twisted nematic mode (TN mode) (Reference 3: Japanese Patent No. 3315476).
In a projection type liquid crystal display device, in order to solve a problem of deterioration of a polarizing plate, a structure in which two or more linear polarizing plates are stacked in a state in which their absorption axes are parallel to each other has been proposed and high display quality can be obtained by the structure (Reference 4: Japanese Published Patent Application No. 2003-172819).
As a flat panel display except for a liquid crystal display device, there is a display device including an electroluminescent element. It can be thin because the electroluminescent element is a self-light emitting element and it does not require a light irradiation means such as a backlight. Further, a display device including an electroluminescent element has an advantage that response speed is higher and dependence on a viewing angle is less than a liquid crystal display device.
A structure in which a polarizing plate or a circularly polarizing plate is provided is also proposed with respect to a display device including an electroluminescent element (Reference 5: Japanese Patent No. 2761453, and Reference 6: Japanese Patent No. 3174367).
As a structure of a display device including an electroluminescent element, a structure in which light emitted from a light emitting element interposed between light-transmitting substrates can be observed on an anode substrate side and on a cathode substrate side (Reference 7: Japanese Published Patent Application No. H10-255976).
A method for increasing a contrast ratio by using three polarizing plates as described in Reference 1 can be realized by using inexpensive polarizing plates; however, it is difficult to perform display with a higher contrast ratio by the method. Although stacking an additional polarizing plate increases a contrast ratio, it cannot suppress slight light leakage. This is because light absorptance changes depending on wavelength of light, that is, absorptance in a specific wavelength region is lower than that in another wavelength region. In other words, light is hardly absorbed only in the specific region. The same kind of polarizing plates are generally used; therefore, even if polarizing plates are stacked to improve contrast, a wavelength region in which light is not easily absorbed remains as it is. This causes the slight light leakage. The light leakage prevents a contrast ratio from being improved.